Constructor in java is used to create the instance of the class. Constructors are almost similar to methods

1. Constructor in Java
P.NAGARAJU
Asst. Professor

2. super keyword in java
 The super keyword in java is a reference variable that is used to refer immediate
parent class object.
 Whenever you create the instance of subclass, an instance of parent class is created
implicitly i.e. referred by super reference variable.
Usage of java super Keyword
 super is used to refer immediate parent class instance variable.
 super() is used to invoke immediate parent class constructor.
 super is used to invoke immediate parent class method.

3. super is used to refer immediate parent class instance variable.
Problem without super keyword
class Vehicle{
int speed=50;
}
class Bike3 extends Vehicle{
int speed=100;
void display(){
System.out.println(speed);//will print speed of Bike
}
public static void main(String args[]){
Bike3 b=new Bike3();
b.display();
}
}
 In the above example Vehicle and Bike both class have a common property
speed.
 Instance variable of current class is refered by instance bydefault.

4. Solution by super keyword
//example of super keyword
class Vehicle{
int speed=50;
}
class Bike4 extends Vehicle{
int speed=100;
void display(){
System.out.println(super.speed);//will print speed of Vehicle now
}
public static void main(String args[]){
Bike4 b=new Bike4();
b.display();
}
}
Output:50

5. super is used to invoke parent class constructor.
The super keyword can also be used to invoke the parent class constructor as given
below:
class Vehicle{
Vehicle(){System.out.println("Vehicle is created");}
}
class Bike5 extends Vehicle{
Bike5(){
super();//will invoke parent class constructor
System.out.println("Bike is created");
}
public static void main(String args[]){
Bike5 b=new Bike5();
}
}
Output: Vehicle is created
Bike is created

6. super can be used to invoke parent class method
1.The super keyword can also be used to invoke parent class method.
2.It should be used in case subclass contains the same method as parent class as in
the example given below:
class Person{
void message(){System.out.println("welcome");}
}
class Student16 extends Person{
void message(){System.out.println("welcome to java");}
void display(){
message();//will invoke current class message() method
super.message();//will invoke parent class message() method
}
public static void main(String args[]){
Student16 s=new Student16();
s.display();
}
}

7. This Keyword in JAVA
In java, this is a reference variable that refers to the current object.
Usage of java this keyword
Here is given the 6 usage of java this keyword.
 this keyword can be used to refer current class instance variable.
 this() can be used to invoke current class constructor.
 this keyword can be used to invoke current class method (implicitly)
 this can be passed as an argument in the method call.
 this can be passed as argument in the constructor call.
 this keyword can also be used to return the current class instance.

8. The this keyword can be used to refer current class instance variable.
If there is ambiguity between the instance variable and parameter, this keyword
resolves the problem of ambiguity.
Ex:
class Student10{
int id;
String name;
Student10(int id,String name){
id = id;
name = name;
}
void display(){System.out.println(id+" "+name);}
public static void main(String args[]){
Student10 s1 = new Student10(111,"Karan");
Student10 s2 = new Student10(321,"Aryan");
s1.display();
s2.display();
}
}

9. Cont…….
In the above example, parameter (formal arguments) and instance variables are same that
is why we are using this keyword to distinguish between local variable and instance
variable.
Ex:
class Student11{
int id;
String name;
Student11(int id,String name){
this.id = id;
this.name = name; }
void display()
{
System.out.println(id+" "+name);
}
public static void main(String args[]){
Student11 s1 = new Student11(111,"Karan");
Student11 s2 = new Student11(222,"Aryan");
s1.display();
s2.display(); } }

10. this() can be used to invoked current class constructor.
The this() constructor call can be used to invoke the current class constructor
(constructor chaining). This approach is better if you have many constructors in the
class and want to reuse that constructor.
The this() constructor call should be used to reuse the constructor in the
constructor. It maintains the chain between the constructors i.e. it is used for
constructor chaining.

11. Ex:
class Student13{
int id;
String name;
Student13(){
System.out.println("default constructor is invoked");}
Student13(int id,String name){
this ();//it is used to invoked current class constructor.
this.id = id; this.name = name;
}
void display(){
System.out.println(id+" "+name);}
public static void main(String args[]){
Student13 e1 = new Student13(111,"karan");
Student13 e2 = new Student13(222,"Aryan");
e1.display();
e2.display();
} }

12. The this keyword can be used to invoke current class method (implicitly).
You may invoke the method of the current class by using the this keyword. If you
don't use the this keyword, compiler automatically adds this keyword while invoking
the method.
Example:
class S{
void m(){
System.out.println("method is invoked");
}
void n(){
this.m();//no need because compiler does it for you.
}
void p(){
n();//complier will add this to invoke n() method as this.n()
}
public static void main(String args[]){
S s1 = new S();
s1.p();
}
}
Output:method is invoked

13. this keyword can be passed as an argument in the method.
The this keyword can also be passed as an argument in the method. It is mainly used in the
event handling.
Example:
class S2{
void m(S2 obj){
System.out.println("method is invoked");
}
void p(){
m(this);
}
public static void main(String args[]){
S2 s1 = new S2();
s1.p();
}
}
Output:method is invoked

14. Introduction
Constructor in java is a special type of method that is used to initialize the object.
Java constructor is invoked at the time of object creation. It constructs the values i.e.
provides data for the object that is why it is known as constructor.
Rules for creating java constructor
There are basically two rules defined for the constructor.
 Constructor name must be same as its class name
 Constructor must have no explicit return type
Types of java constructors
There are two types of constructors:
1.Default constructor (no-arg constructor)
2.Parameterized constructor

15. Java Default Constructor
A constructor that have no parameter is known as default constructor.
Syntax of default constructor:
<class_name>()
{
}
Example of default constructor
class Bike1{
Bike1(){System.out.println("Bike is created");}
public static void main(String args[]){
Bike1 b=new Bike1();
}
}

16. Purpose of default constructor
Default constructor provides the default values to the object like 0, null etc. depending
on the type.
EX:
class Student3
{
int id;
String name;
void display(){System.out.println(id+" "+name);}
public static void main(String args[]){
Student3 s1=new Student3();
Student3 s2=new Student3();
s1.display();
s2.display();
}
}

17. Java parameterized constructor
 A constructor that have parameters is known as parameterized constructor.
 Parameterized constructor is used to provide different values to the distinct objects.
EX:
class Student4{
int id;
String name;
Student4(int i,String n){
id = i;
name = n;
}
void display(){System.out.println(id+" "+name);}
public static void main(String args[]){
Student4 s1 = new Student4(111,"Karan");
Student4 s2 = new Student4(222,"Aryan");
s1.display();
s2.display();
}
}

18. Constructor Overloading in Java
 Constructor overloading is a technique in Java in which a class can have any number
of constructors that differ in parameter lists.
 The compiler differentiates these constructors by taking into account the number of
parameters in the list and their type.

19. Example:
class Student5{
int id;
String name;
int age;
Student5(int i,String n){
id = i;
name = n;
}
Student5(int i,String n,int a){
id = i;
name = n;
age=a;
}
void display(){System.out.println(id+" "+name+" "+age);}
public static void main(String args[]){
Student5 s1 = new Student5(111,"Karan");
Student5 s2 = new Student5(222,"Aryan",25);
s1.display();
s2.display();
}
}

20. Java Copy Constructor
There is no copy constructor in java. But, we can copy the values of one object to
another like copy constructor in C++.
There are many ways to copy the values of one object into another in java. They are:
 By constructor
 By assigning the values of one object into another
 By clone() method of Object class

21. Example:
class Student6{
int id;
String name;
Student6(int i,String n){
id = i;
name = n;
}
Student6(Student6 s){
id = s.id;
name =s.name;
}
void display(){System.out.println(id+" "+name);}
public static void main(String args[]){
Student6 s1 = new Student6(111,"Karan");
Student6 s2 = new Student6(s1);
s1.display();
s2.display();
}
}